Offline-safe Calibration and IMF SDMX 3.0 refresh

[SeaCelo]

Make calibration offline-safe and refresh the IMF GFS data source.

Calibration (calibrate.py)

• API updates are off by default
• Each data source fails independently with a warning instead of crashing the whole calibration
• Offline mode returns sensible defaults for the single-sector case

IMF GFS (macro_params.py)

• Moves to the current SDMX 3.0 endpoint
• Ethiopia uses the budgetary central government sector; other countries default to general government
• Falls back to the last complete year if the target year is missing
• Optional argument to save the raw pull to CSV

SAM loading (input_output.py)

• Returns None on failure; downstream helpers raise a clear error if SAM is unavailable

Documentation (docs/book/content/calibration/macro.md)

• Adds the GFS formulas for alpha_T and alpha_G and explains the sector choice for Ethiopia

Tests: new test_calibrate.py and test_input_output.py; test_macro_params.py rewritten with mocked HTTP.

[SeaCelo]

This mirrors the ZAF changes. Ready for review
@jdebacker @rickecon